1. Field of the Invention
The present invention relates in general to improvements in a spin-polarized electron beam emitting element, an electron source including such an element, and a device including such an electron source. More particularly, the present invention relates to such element and source which assure high quantum efficiency and beam density of emitted electrons, and to such a device which is compact and easy to handle.
2. Discussion of the Related Art
A spin-polarized electron beam in which the electrons have their spins aligned in one of two possible directions is utilized as effective means for studying or investigating the magnetic properties in the nucleus of atoms in the field of experiment on a high-energy elementary or fundamental particle, and the magnetic properties near the surface of a material in the field of material science. Spin-polarized electrons are photo-emitted from an electron emitting element including a semiconductor opto-electronic layer, by laser-exciting the opto-electronic layer. This semiconductor opto-electronic layer has energy level splitting or difference in the valence band, due to a lattice mismatch with respect to another semiconductor layer on which the opto-electronic layer is grown.
Described more specifically, the electron emitting element includes a heterojunction structure consisting of a GaAs.sub.1-x P.sub.x (x&gt;0) semiconductor layer, and a so-called strained GaAs semiconductor layer grown by epitaxy on the GaAs.sub.1-x P.sub.x semiconductor layer. The strained GaAs layer as the opto-electronic layer indicated above has a smaller band gap than the GaAs.sub.1-x P.sub.x layer, and a lattice constant slightly different from that of the GaAs.sub.1-x P.sub.x layer. A lattice mismatch between the two layers constituting a heterostructure gives the GaAs layer a strain which causes splitting of the valence band, that is, a strain-dependent energy level splitting or difference of the heavy-hole and light-hole bands (sub-bands) of the valence band relative to the conduction band. The energy splitting or difference makes it possible to preferentially excite one of the heavy-and light-hole sub-bands of the valence band which has the higher energy level, namely, which has the smaller energy gap with respect to the conduction band. Therefore, by suitably tuning the excitation laser energy incident upon the GaAs semiconductor layer so as to excite one of the sub-bands of its valence band, the electrons emitted from the GaAs layer can be spin-polarized in one of the two opposite directions which corresponds to the excited sub-band.
Thus, the use of a strained opto-electronic layer assures improved spin polarization of electrons of a beam photoemitted from an electron emitting element. The strained GaAs semiconductor layer indicated above may be replaced by a strained GaAs.sub.1-y P.sub.y (y&gt;0) semiconductor layer or other strained compound semiconductor layer as disclosed in Japanese Patent Application No. 4-196245 filed in the name of the assignee of the present application, or by a semiconductor of chalcopyrite type which has a split valence band by nature.
However, the conventional spin-polarized electron emitting element does not assure spin polarization of emitted electrons with sufficiently high quantum efficiency (QE), that is, suffers from a small number of electrons emitted per excitation laser input. Accordingly, the conventional emitting element requires a considerably long period of laser excitation to obtain a sufficient amount of electron beam intensity, making it difficult to achieve real-time observation or investigation of a magnetic domain of a material, for instance. Although an increase in the thickness of the opto-electronic layer will increase the number of photoemitted electrons and the quantum efficiency, this solution results in a variation in the spin polarization of the emitted electrons due to scattering within the semiconductor, unfavorably leading to reduced polarization percent.
Further, the conventional spin-polarized electron emitting element is not capable of adjusting the electron beam intensity, and therefore suffers from difficulty in observing a magnetic domain near the surface of a magnetic material, for example, while changing a relationship between the intensity of the emitted electron beam and an image observed of the material surface, with the electron beam intensity changed as a parameter.
In observing the magnetic domain on a magnetic material surface by exposure thereof to a spin-polarized electron beam photoemitted from the electron emitting element described above, the emitting element is disposed in a vacuum chamber, and the opto-electronic layer of the element is irradiated by an excitation laser generated from a suitable laser source, to photoemit a spin-polarized electron beam. The photoemitted electron beam is conducted or directed to the surface of the specimen, a magnetic material, through a transmission device connected to the vacuum chamber, as in an apparatus disclosed in the above-identified Japanese Patent Application, which is adapted to measure the spin polarization percent of a photoemitted electron beam.